Work around qemu static deadlock

Workaround issue pmbootstrap#1715 (closed).

This adds jemalloc package and brings back qemu package. These packages are compiled for x86 and x86_64 only. The binfmt_misc chroots on arm hosts are untested.

I tried to run an x86_64 pmbootstrap chroot on an arm device (ouya) but binfmt_misc doesn't seem to work. The /proc/sys/fs/binfmt_misc directory is empty.